Hybrid Materials: Bringing Together the Best Features

Scientists combine the properties of organic and inorganic materials to achieve advanced technologies

Evan Lafalce

Hybrid perovskite sheets with a controllable number of inorganic layers. The black diamonds represent metallic lead while the orange circles represent halogen atoms such as iodide. The layered lead-iodide sheets are capped on the top and bottom by organic molecules. Modifying the choice of organic molecules used can control the number of inorganic layers, n, which in turn can modify properties such as light emission and solar electricity generation. Image courtesy of the American Chemical Society, copyright 2018 (see Chen et al. in More Information)

Scientists typically sort materials into two categories—organic and inorganic. Organic materials are composed primarily of light atoms, such as carbon, nitrogen, and oxygen, and in this way are similar to the materials of which plants, skin, and muscles are composed. Inorganic materials, on the other hand, contain much heavier atoms, such as silver, gold, and copper, that make up metals used in conducting electricity, as well as silicon, germanium, and indium that make up semiconductors used in solar cells and light-emitting diodes and computers.

Light-weight organic materials, typically, are known for their abundance, flexibility, and chemical reactivity. Inorganic materials are commonly known for their strength, stability, and useful electronic functions. While we’ve been using and studying hybrid materials for ages (including paints and delicate porcelains), researchers are focusing on a new generation of hybrid materials where the mixing occurs on the atomic and molecular levels. The inorganic parts are mainly responsible for the primary functions, such as the absorption of light and transport of electricity in a solar cell, the stimulation of chemical reactions that produce fuels, or the storage of energy in batteries. The organic parts, on the other hand, allow for easier means of producing multiple shapes and structures, and to change the properties of the materials in precise and subtle ways. The combination of the two material classes then creates a wide range of hybrid materials that scientists can tailor to achieve specific functions to create technologies and solve society’s energy-related problems.

At the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center, researchers are developing a particularly exciting new class of hybrid materials known as hybrid perovskites. Hybrid perovskites contain an inorganic core inside an organic cage. The inorganic parts provide properties like the ability to absorb light and produce electrical current for solar energy production, or the ability to emit light for energy-saving light sources known as light-emitting diodes, or LEDs. Meanwhile, scientists can use the organic molecules to produce many different structures of hybrid perovskites. Not only that, but the different structures can be grown relatively easily from liquid solutions, which makes them less expensive and more energy efficient. For instance, by simply changing the organic component in the solution, these scientists create large 3-D crystals or thin sheets where the organic and inorganic parts are stacked in layers. This is a great advantage over purely inorganic materials that require sophisticated and expensive energy-consuming techniques to grow. Further, changing the structure using different organic molecules also changes the properties of the perovskite, allowing researchers to optimize material properties for specific energy applications.

In a recent example, scientists at CHOISE looked at how the ratio of the number of organic layers to inorganic layers in the thin sheets determined the electrical current through the perovskite. When the organic and inorganic layers were alternating in a one-to-one ratio, they found that the current would tend to convert to light emission, suggesting the material will be better suited for use in LEDs. When they increased the number of inorganic layers between the organic ones, the ability to conduct electricity improved, making these materials better suited for generating solar electricity.

Hybrid cages for catalysis

Catalysts are substances that speed up chemical reactions. Development of catalysts that increase the efficiency of reactions to produce renewable fuels is important to keep up with growing energy demand, and is one of the primary goals of the Inorganometallic Catalyst Design Center (ICDC). ICDC researchers are investigating the use of metal-organic frameworks as hybrid materials that serve as catalysts to produce renewable fuels. Metal-organic frameworks are tiny cage-like structures formed by the hybrid combination of inorganic metals linked together by organic molecules and provide countless structures varying in size, shape, and density, all of which can be used to tune the efficiency of the catalyst and determine what specific reactions will occur.

Metal-organic frameworks consist of metal clusters linked to organic molecules. The choice of the organic molecule allows the molecules to be adapted into cage-like structures like the one on the right. Scientists can tune the size, shape, and porosity to catalyze different reactions and produce different fuels. Image courtesy of the American Chemical Society, copyright 2019 (see Wei et al. in More Information)

In a recent study, the scientists with ICDC showed that they could control the purity of the samples through the choice of different organic molecules that link the metals together. “One of the unique challenges of combining organic and inorganic materials for catalysis is the synthesis of stable materials. Therefore, a careful selection of the ‘pieces’ is of pivotal importance in synthesis,” said Carlo A. Gaggioli, an ICDC postdoctoral researcher.

Control over sample purity is important because the imperfections, or defects, that are formed during synthesis can affect the catalytic ability, though not necessarily in a negative manner. The defects can be useful for catalysis, but knowing their number and type is challenging. Being able to accurately count and classify the defects is the key to rational control of the properties of metal-organic frameworks for catalytic applications. The study demonstrated just such an ability by a combination of experimental and computational techniques. Further, the cage-like form of metal-organic frameworks means they can trap the molecules that take part in the reaction, which can be used to provide higher activity or select a specific reaction to occur. The researchers also achieved specific protocols for increasing the size of the cages in these hybrid complexes, providing a means to controllably catalyze different fuels

“Another challenge is to understand the catalytic mechanisms in hybrid materials, which can be more complicated with respect to pure inorganic or organic materials,” said Gaggioli. “At the same time, the interplay of properties of the inorganic and organic parts can be extremely advantageous. For example, the exploitation of cage size, which can be rationally modified to achieve selective catalysis by excluding the formation of molecules that are too big to fit in the cage is an enormous advantage.”

A hybrid approach for holey energy storage sheets

Combining the benefits of organic and inorganic materials is an approach also used at the Center for Mesoscale Transport Properties (m2mt), an Energy Frontier Research Center focused on developing state-of-the-art energy storage materials and systems, such as batteries. Energy storage materials require strength and stability to transport ions. The scientists at m2mt have found ingenious ways to improve energy storage capacities by using organic materials during their preparation of materials or by using them as interfaces in devices.

Constructing holey nanosheets. Nanoparticles are arranged on a hexagonal graphene-oxide (GO) template and converted into a 2-D holey transition metal oxide (TMO) nanosheet with the help of organic pluronic copolymers. The molecular weight of the copolymer allows the appropriate size of holes to be set in the nanosheet, aiding in the energy storage capabilities of this material. Image courtesy of the American Chemical Society, copyright 2018 (see Peng et al. in More Information)

In a recent breakthrough, they reported the ability to use organic molecules called pluronic copolymers to distribute tiny inorganic particles evenly on a structural template. Once the nanoparticles were correctly distributed, the team converted them into a flat sheet by a chemical process. The resulting sheets contain holes that are useful for storing electrical charge that may later be used to produce energy, like a battery. Even more impressive, the researchers showed that the size of the molecules making up the pluronic polymers had a direct influence on the size of the holes. Thus, the holes provide a means to control the energy storage capability of the nanosheets.

Armed with the knowledge obtained from years of independent research into organic materials and inorganic materials separately, the forefront of energy materials research now can benefit from the combination of both material classes in the form of hybrids. Researchers at Energy Frontier Research Centers across the nation are actively exploiting these new synthetic material capabilities to pursue solutions to future energy challenges, from solar cells and power-saving lighting to enhanced production of fuels and energy storage materials.

Acknowledgments

Chen et al.This work was supported by the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences. The method development was supported by the National Science Foundation. Y Yang acknowledges support from the Ohio Research Scholar Program. This research used the resources of the National Energy Research Scientific Computing Center, which is supported by the DOE Office of Science.

Wei et al. This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences.

Peng et al. The authors acknowledge the Center for Mesoscale Transport Properties, an Energy Frontier Research Center from the Department of Energy (DOE), Office of Science, Basic Energy Sciences for financial support of this project. They also acknowledge the Transmission Electron Microscopy Facility in the Central Microscopy Imaging Center (C-MIC) at Stony Brook University, Stony Brook, New York, for their contribution towards the transmission electron microscopy preparation and data collection. This research used resources of the Center for Functional Nanomaterials, which is a DOE Office of Science user facility, at Brookhaven National Laboratory. A.M.B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program.

About the author(s):

Evan Lafalce is a research assistant professor at the University of Utah, Department of Physics & Astronomy, working under Z. Valy Vardeny. He is a member of the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), where he studies the optical, electronic, and spin properties of hybrid organic-inorganic lead-halide perovskites. He obtained his Ph.D. in applied physics from the University of South Florida in Tampa, where he characterized organic solar cells and organic solar cell materials. He was born in Baltimore, Maryland.